Self-setting calcium phosphate cements and methods for preparing and using them

ABSTRACT

The invention includes methods and compositions relating to calcium phosphate cements, which self-harden substantially to hydroxyapatite at ambient temperature when in contact with an aqueous medium. More specifically the cements comprise a combination of one or more sparingly soluble calcium phosphates other than tetracalcium phosphate with an aqueous solution adjusted with a base to maintain a pH of about 12.5 or above and having sufficient dissolved phosphate salt to yield a solution mixture with phosphate concentration equal to or greater than about 0.2 mol/L.

This invention was made in the course of research partially supported bya government grant from the National Institute of Dental Research (GrantNo. DE 05030). The U.S. government has certain rights in the invention.

This is a continuation of Ser. No. 08/658,978 filed Jun. 4, 1996 whichis a continuation of Ser. No. 08/126,502 filed Sep. 24, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to calcium phosphate compositions, includingcements and pastes, and to methods for making and using them. In itspreferred form, the invention relates to methods for makinghydroxyapatite forming cements and pastes without the necessity foremploying tetracalcium phosphate as a precursor.

2. Description of Related Art

In the area of dental cements, the prior art includes a number ofcompounds. Some such cements, however, irritate the pulp and areunsuitable for applications where the cement must come in contact withexposed pulp. Guide to Dental Materials and Devices, 7th Ed. (ADA 1974)p. 49. One solution to this problem is a cement made of materialssimilar in composition to tooth and bone mineral, since this would notirritate the living tissue. It has been known for some time thathydroxyapatite materials have the basic properties of human bones andteeth.

The use of β-Ca₃ (PO₄)₂ was suggested for pulp capping in Driskell etal., "Development of Ceramic and Ceramic Composite Devices forMaxillofacial Application", J. Biomed. Mat. Res. 6: 345-361 (1972); andthe use of Ca₄ (PO₄)₂ O was suggested by Brown and Chow in IADR AbstractNo. 120, J. Dent. Res. 54: 74 (1975), as a possible pulp capping agent.As described in the latter, Ca₄ (PO₄)₂ O hydrolyzes to hydroxyapatite.Such single calcium phosphate cements are incapable of setting to a hardconsistency, however, without an additional source of calcium.

Though U.S. Pat. No. 3,913,229 (Driskell et al.) discloses putty-likepastes containing α-Ca₃ (PO₄)₂, β-Ca₃ (PO₄)₂, CaHPO₄ and mixturesthereof as pulp capping, root canal, and tooth replanting materials, itis believed that none of these pastes hardens into a cement.

Experience with calcium-based implants for the replacement of skeletaltissue has also existed for many years. Most of these implants have beenin the form of prefabricated, sintered hydroxyapatite in either granuleor block forms. These preparations have several drawbacks, including alimited ability to conform to skeletal defects, particularly in the caseof blocks; inadequate structural integrity of granules (which do notbond together), and difficulty in modeling the implant to the shape ofmissing skeletal tissue with both blocks and granules. The block form ofhydroxyapatite provides structural support, but among othercomplications, must be held in place by mechanical means, which greatlylimits its use and its cosmetic results; and it is very difficult to sawa shape such that it fits the patient's individual defect. The granularform produces cosmetically better results, but has a very limitedstructural stability and is difficult to contain during and after asurgical procedure. In general, all of these products are ceramics,produced by high temperature sintering, and are not individuallycrystalline, but rather have their crystal boundaries fused together.These ceramic-type materials are in general functionally biologicallynon-absorbable (having an absorption rate generally not exceeding on theorder of 1% per year).

A porous, non-resorbable material based on coral allows intergrowth withbone, but ultimately becomes only approximately 20% bone with theremaining 80% subsisting as scar tissue. HA RESORB made by Osteogen is aform of absorbable hydroxyapatite, but is not a cement. It is granularand not adhesive. HA RESORB is loosely rather than adhesively packedinto place. For large uses, it is replaced by bone too quickly. In thedental materials market, HAPSET is a composition of calcium phosphategranules and cementable plaster of Paris (calcium sulfate). Thismaterial is not truly a hydroxyapatite and contains too much calciumsulfate for most biological uses. The calcium sulfate component of sucha composition is resorbable, but not the calcium phosphate granules.

In sum, the commercially available hydroxyapatite materials are ingeneral not resorbable with accompanying replacement by bone, and arenot self-setting (self-hardening) cements.

The patent literature, does, however, describe at least one class ofcalcium phosphate cement compositions which are precursors for theformation of hydroxyapatite and are biologically compatible, and havetwo unique properties that are not attainable in other calcium phosphatebiomaterials: (1) self-hardening to form a mass with sufficient strengthfor many medical and dental applications, and (2) when implanted inbone, the cement resorbs slowly and is completely replaced by new boneformation with no loss in the volume or integrity of the tissue thatreceives the implant. See U.S. Pat. Nos. Re. 33,221 and Re 33,161 toBrown and Chow, which teach preparation of calcium phosphateremineralization compositions and of a finely crystalline, non-ceramic,gradually resorbable hydroxyapatite cement based on the same calciumphosphate composition.

A virtually identical calcium phosphate system which consists oftetracalcium phosphate (TTCP) and monocalcium phosphate (MCP) or itsmonohydrate form (MCPM) was described by Constantz et al. (U.S. Pat.Nos. 5,053,212 and 5,129,905); This cement system is believed to involveconversion of the MCP to dicalcium phosphate which reacts with TTCP andforms hydroxyapatite (HA), the major mineral component of teeth andbone, as the end product.

Constantz et al. U.S. Pat. Nos. 4,880,610 and 5,047,031 describe anothercement system that consists of a mixture of solid phosphoric acidcrystals, calcium carbonate, and calcium hydroxide as the cement powderand a 7.4 mol/L NaOH solution (4.5 g NaOH in 15 mL of water) as thecement liquid. Data on the physical and chemical properties (compressivestrength, hardening time, nature of end product, pH of the cement fluid,heat of mixing etc.) of this cement have not been located in the patentor scientific literature.

The major components of the calcium phosphate remineralizing slurries,pastes and cements taught in U.S. Pat. Nos. Re. 33,221 and Re. 33,161are preferably tetracalcium phosphate (Ca₄ (PO₄)₂ O), and at least oneother sparingly soluble calcium phosphate, preferably dicalciumphosphate anhydrous (CaHPO₄), or dicalcium phosphate dihydrate(CaHPO₄.2H₂ O). These react in an aqueous environment to formhydroxyapatite, the principal mineral in teeth and bones, as the finalproduct. Because of the apatitic nature of the set cement, it is highlycompatible with soft and hard tissues. This material, if appliedintraoperatively as a paste, subsequently sets to a structurally stableimplant composed of microporous hydroxyapatite.

SUMMARY OF THE INVENTION

The materials of Brown and Chow U.S. Pat. Nos. Re. 33,221 and Re.33,161, while highly useful, are relatively expensive because of thecost of tetracalcium phosphate. Applicants have discovered that use oftetracalcium phosphate may be avoided in the preparation of the calciumphosphate cement composition, while still yielding a product whichself-sets to hydroxyapatite, provided: (1) a high phosphateconcentration is maintained in the precursor slurry solution, and/or (2)the pH is substantially elevated above neutral. In particular,applicants have discovered that the use of a calcium phosphate precursorslurry without TTCP, which has a phosphate concentration in the solutionat or above about 0.2 mol/L and/or a pH in the range of from about 12.5to about 14 results in a cement which sets reliably and quickly to HAwithout the use of tetracalcium phosphate in the precursor mixture.Applicants have also discovered that the rate of conversion to HA isunchanged for calcium phosphate cements prepared by this method andsetting times are improved. The setting rate can be adjusted for variousend uses, and may be quite rapid if desired. The inventivehydroxyapatite cement is believed to be both biocompatible andresorbable (biodegradable) with bone replacement when in contact withliving bone.

The invention includes a method for preparing calcium phosphate cementcompositions, which self-harden substantially to hydroxyapatite atambient temperature when in contact with an aqueous medium, comprisingcombining one or more sparingly soluble calcium phosphates other thantetracalcium phosphate with an aqueous phase having sufficient dissolvedphosphate salt to yield a solution mixture with phosphate concentrationequal to or greater than about 0.2 mol/L. The invention further includesa method for preparing calcium phosphate cement compositions, whichself-harden substantially to hydroxyapatite at ambient temperature whenin contact with an aqueous medium, comprising combining one or moresparingly soluble calcium phosphates other than tetracalcium phosphatewith an aqueous phase such that the pH of the solution followingcombination is initially in the range of about 12.5 to about 14.Alternatively, the inventive method may involve utilization of boththese sets of conditions simultaneously.

The invention further contemplates the improved calcium phosphate cementmixtures prepared by this method, the cement component(s) provided tothe user in a pre-manufactured kit or delivery device, the methods ofusing the improved cement, and the biological implants made from thecement. A self-hardening industrial cement is also contemplated.

The techniques previously commercially available for repair of cranio-and maxillofacial defects, periodontal defects, bone fractures and otherdental and orthopedic defects which could not be successfullyself-healed relied heavily on the use of metallic and ceramic insertsand prostheses which might remain indefinitely as foreign objects in thebody of a human or veterinary patient. As such, these prior arttechniques suffered from a host of related problems, including possiblerejection, sites for infection, structural failure, injury toneighboring tissue and the like. Metals are difficult to shape and arehampered by problems such as infection and corrosion. Polymers such assilicone, PROPLAST, or methyl-methacrylate are encapsulated by scartissue resulting in significant rates of implant infection and/orextrusion. Biologic materials, such as autogenous bone grafts, may causedonor site morbidity, may suffer from significant post-implantationresorption, and are difficult to accurately conform to skeletal defects.U.S. Pat. Nos. Re. 33,221 and Re. 33,161 and pending application Ser.No. 08/030,709 offer the alternative of a hydroxyapatite-forming calciumphosphate cement which is biocompatible and which, when implanted incontact with living bone, is largely if not completely replaced by newbone formation, with no significant loss in the volume or integrity ofthe tissue that receives the implant. The above-cited patents to Brownand Chow teach the preparation of bioresorbable cements preferably fromcombinations of tetracalcium phosphate (Ca₄ (PO₄)₂ O) and anothersparingly soluble calcium phosphate compound, preferably dicalciumphosphate or dicalcium phosphate dihydrate. The above-cited application,Ser. No. 08/030,709 teaches the preparation of calcium deficient HA fromspecially prepared and stored tetracalcium phosphate and anothersparingly soluble calcium phosphate compound.

The inventive method described and claimed herein results in cementswhich are characterized by many important advantages as compared withtraditional hydroxyapatite materials, and which are significantly lessexpensive than those of Brown and Chow and of Chow and Takagi, citedabove.

An object and advantage of the present invention is the provision of amethod for preparing a self-setting calcium phosphate cement whichresults in particularly rapid setting of the cement.

A further object and advantage of the invention is the provision of amethod for making a calcium phosphate cement which employs lessexpensive starting materials than prior art cements of comparableproperties.

Further objects and advantages of the invention include the cementmixtures prepared by the inventive method, the methods of using thecements, and the products made from them embodying the characteristicsset forth above.

Additional advantageous attributes of the inventive cement include thefollowing:

The cement is easy to use and can be readily modeled to accuratelyreconstruct bony cavities and missing bone and to recreate contourdefects of relatively non-stress bearing skeletal bone.

The cement or paste consistency enables the hydroxyapatite to conformexactly to the contours of a defect. The cement can be applied to adefect, e.g., with a spatula, can be molded and sculpted, and will holdits shape satisfactorily until set.

The inventive cement sets at ambient temperature, e.g., room or bodytemperature, unlike the ceramic-type calcium phosphate cements whichmust be sintered at high temperature in a process that fuses individualhydroxyapatite crystals to each other. The cement setting reaction forthe inventive material is isothermic (negligible heat is generated) andthus does not result in heat-generated necrosis of any of theneighboring tissue.

The inventive cement can be easily sculpted in vivo (intraoperatively)even after setting. When applied to clean, non-infected bone, the cementadheres to the bone, thereby greatly increasing its possibleapplications. Additionally, the inventive cements of the presentinvention are especially useful in in vitro applications, including thecreation of implants. In general, the cement can be adapted to manyapplications.

The instant cement represents a highly biocompatible tissue substituteprecursor or synthetic implant material for skeletal reconstruction.This biocompatibility stems from the fact that calcium phosphate existsin bone in the form of hydroxyapatite, and it is therefore a chemicallynatural material. Basically, the inventive cement is regarded by thebody as a native material; the elevated pH which characterizes certainforms of the cement rapidly diminishes to neutral and therefore presentslittle threat of injury to the surrounding tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show the solubility phase diagram of the 3-componentsystem Ca(OH)₂ --H₃ PO₄ --H₂ O, in which both the solid phase and thesaturated solution contain only those ions or non-charged species thatare derived from the three components, Ca(OH)₂, H₃ PO₄ and H₂ O.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The complete disclosures of U.S. Pat. Nos. Re. 33,221 and Re. 33,161 andpending application Ser. No. 08/030,709 are expressly incorporatedherein by reference.

The inventive hydroxyapatite cement is an alternative to theself-setting cement of calcium phosphate compounds developed by Brownand Chow and referenced above. The preferred major components of thecalcium phosphate cement of Brown and Chow are tetracalcium phosphate(TTCP) and dicalcium phosphate anhydrous (DCPA) or dicalcium phosphatedihydrate (DCPD). These react in an aqueous environment to formhydroxyapatite (HA), the principal mineral component of teeth and bones,as the final product.

The chemical reaction that occurs during the setting of the TTCP-DCPA(or TTCP-DCPD) cement described in Brown and Chow (Re. U.S. Pat. Nos.33,161 and 33,221) can be represented by the following equation:##STR1## As described later, the choice of TTCP and DCPA (DCPD) as thecement ingredients is important because the solubilities of TTCP andDCPA are such that the cement fluid, which is approximately saturatedwith respect to both salts, would have a slightly above neutral pH(about 7.5 to 8.5) and sufficiently high calcium (Ca) and phosphate (P)concentrations so that the solution is substantially supersaturated withrespect to HA. Rapid HA formation and concomitant dissolution of bothcement ingredients, TTCP and DCPA, lead to the hardening of the cementordinarily within 30 minutes or less.

FIGS. 1a and 1b show the solubility phase diagram of the three-componentsystem, Ca(OH)₂ --H₃ PO₄ --H₂ O, in which both the solid phase and thesaturated solution contain only those ions or non-charged species thatare derived from the three components, Ca(OH)₂, H₃ PO₄, and H₂ O. Eachcurve, or solubility isotherm, in the diagram represents the composition(in terms of the pH and calcium concentration in FIG. 1a and the pH andphosphate concentration in FIG. 1b) of a series of solutions that areall saturated with respect to a given salt. A calcium phosphate saltthat has a solubility isotherm that lies below that of another salt isless soluble (and more stable) than the other salt. It can be seen inthe figures that among all calcium phosphate salts, HA is the leastsoluble in a wide range of solution pH's, ranging approximately from 4.5to 14. Thus, within this pH range, any other calcium phosphate ormixture of calcium phosphates has the tendency to dissolve andreprecipitate as HA. However, in general the rate of HA formation isvery slow such that a slurry of DCPD, DCPA, octacalcium phosphate (OCP),amorphous calcium phosphate (ACP), α-tricalcium phosphate (α-TCP),β-tricalcium phosphate (β-TCP), or a mixture of these salts does notproduce a setting cement or act as an effective remineralizing agent.

It has now been discovered that HA formation in calcium phosphateslurries can be greatly accelerated by one or both of the followingfactors: (1) a high phosphate concentration in the slurry solution, and(2) a high degree of supersaturation with respect to HA produced byraising the solution pH to a level where most of the phosphate is in theform of PO₄ ³⁻ and little phosphate is in the form of H₃ PO₄ or H₂ PO₄⁻.

It is believed that the key to the inventive cement system is to provideeffective means to accelerate the HA formation in the slurry systemsthat do not contain TTCP. This can be achieved by producing andmaintaining a high phosphate concentration (e.g., 0.2 mol/L or higher),a high degree of supersaturation with respect to HA by raising thesolution pH to about 12.5 or above, or both. It is noted that in allcases, the pH of the slurry must be in the range of approximately 4.5 to14, the range in which HA is the most stable, phase. Once this isachieved, rapid HA formation and subsequent cementation can occur inpractically any calcium phosphate slurries. Described below areapproaches to attain high phosphate concentration and high degree of HAsupersaturation in the slurry solutions.

In one embodiment, the invention involves producing and maintaining ahigh phosphate concentration in the calcium phosphate slurry solution.Contrary to what appears to be the most obvious method to achieve thegoal, adding phosphoric acid to a calcium phosphate slurry is not themethod of choice to increase the phosphate concentration to 0.2 mol/L orhigher. This is because as long as the slurry solution remains as athree-component system, Ca(OH)₂ --H₃ PO₄ --H₂ O, the phosphateconcentration is limited by the solubility isotherms of the solid thatis in contact with the solution (FIG. 1b). Thus, after adding 0.2 mol/Lof phosphoric acid, the pH of the slurry would drop to significantlybelow 4.5 initially. As the calcium phosphate crystals dissolve and pHincreases, the phosphate concentration would drop to below 0.1 mol/L asthe pH increases to 4.5 (FIG. 1b). An effective way to increase thephosphate concentration is to add a non-calcium-containing salt ofphosphoric acid that is sufficiently soluble, e.g., Na₃ PO₄, Na₂ HPO₄,or NaH₂ PO₄. Salts of other cations such as K⁺, NH₄ ⁺, etc., would workequally well provided that the cations do not form strong complexes withphosphate to the point of making phosphate unavailable for HAprecipitation.

The reason why a high phosphate concentration can be maintained byadding an appropriate amount of sodium phosphate salt is explained asfollows. In the example where the pH is between 7.2 and 12.7, since thesolution would always have a Na⁺ concentration of 0.4 mol/L (regardlessof the calcium phosphate dissolution and precipitation reactions thatmay occur), the total anion concentration in the solution must be suchthat the anions can provide sufficient negative charges to balance thepositive charges of the Na⁺ ions. Since OH⁻ and phosphate ions are theonly anions present in the solution, and since OH⁻ concentration wouldbe only 0.01 mol/L even at pH of 12, most of the negative charges haveto be provided by the phosphate ions. Further, because in this pH rangethe dominant phosphate species is HPO₄ ²⁻ ; the total phosphateconcentration needed to balance the positive charges of the Na⁺ ionswould be approximately 0.2 mol/L.

A second embodiment of the invention involves increasing the degree ofsupersaturation with respect to HA by increasing the pH to approximately12.5 or above. The pH can be increased by adding a strong base, e.g.,NaOH, KOH, etc. Ca(OH)₂ by itself is insufficient to increase the HAformation significantly, but it needs to be included as a solidcomponent as described later. It is noted that raising the pH to 12.5 orabove also increases the solubility of those calcium phosphate saltsthat have HPO₄ ²⁻ ions in their structures because at pH aboveapproximately 12.7 the dominant phosphate species is the PO₄ ³⁻ ion.Thus, for slurries that contain these salts, a high phosphateconcentration can also be obtained by increasing the pH of the slurrysolution to approximately 12 or above. In contrast, raising the pH wouldnot be effective in increasing the phosphate concentration in slurriesthat contain ACP, α-TCP, or β-TCP.

Regardless of which embodiment of the invention is used, certainconsiderations apply to the cement setting reactions. The formation ofHA and dissolution of the more soluble calcium phosphate compounds areresponsible for the hardening of the cement. Since all calciumphosphates that will be used in the cement starting ingredients haveCa/P molar ratio below 5/3, the Ca/P ratio of HA, an additional sourceof calcium is needed to form HA as the dominant cement end product. Forexample, Ca(OH)₂, CaO and CaCO₃ can be used for this purpose. To achievecomplete conversion to HA, the cement should contain e stoichiometricamounts of the calcium phosphate ingredients according to the settingreaction. Given below are some representative reactions:

    3 CaHPO.sub.4 +2 Ca(OH).sub.2 →Ca.sub.5 (PO.sub.4).sub.3 OH+3H.sub.2 O                                                         (3)

    3 CaHPO.sub.4 +2 CaO→Ca.sub.5 (PO.sub.4).sub.3 OH+H.sub.2 O(4)

    3 CaHPO.sub.4 +2 CaCO.sub.3 →Ca.sub.5 (PO.sub.4).sub.3 OH+H.sub.2 O+2 CO.sub.2                                              (5)

    3 α-Ca.sub.3 (PO.sub.4).sub.2 +Ca(OH).sub.2 →2Ca.sub.5 (PO.sub.4).sub.3 OH                                       (6)

    3 CaHPO.sub.4.2H.sub.2 O+2Ca(OH).sub.2 →Ca.sub.5 (PO.sub.4).sub.3 OH+9 H.sub.2 O                                            (7)

    3 amorphous Ca.sub.3 (PO.sub.4).sub.2 +Ca(OH).sub.2 - - - Ca.sub.5 (PO.sub.4).sub.3 OH                                       (8)

EXAMPLES 1-58

Calcium phosphate cements were made using the methods of the presentinvention as follows:

A calcium phosphate cement powder was prepared by mixing a calciumphosphate salt (substantially free of tetracalcium phosphate) such asDCPA, DCPD, α-TCP or ACP with an additional source of calcium such asCaCO₃, Ca(OH)₂, or CaO. These calcium components may be mixed asreceived from vendors or ground to smaller particle size. Table I belowprovides examples of specifications for particles size of thecomponents.

The calcium phosphate cement of the present invention was made by mixing0.3 gram of calcium phosphate cement powder with 0.075 mL of an aqueoussolution of a non-calcium containing salt of phosphoric acid, withpowder to liquid weight ratio (P/L) of about 3, such as 1 M Na₂ HPO₄(P/L=3), 0.5M Na₂ HPO₄ (P/L=3), 5M (NH₄)₂ HPO₄ (P/L=2.5). 1 N NaOH and4N NaOH solutions were also used to formulate the cement. To allowtesting of the setting time and strength of the cement, the mixture wasspatulated on a glass slab for 30 sec, and placed in a stainless steelmold (6 mm d×3 mm h). The top and bottom surfaces of the mold weretightly covered with glass plates and the mold was placed in a 100%humidity box kept at 37° for 4 hours. The sample was removed from themold and placed in a small amount of water for 20 hours. The diametraltensile strength (DTS) was measured with the use of a Universal TestingMachine (United Calibration Corps Garden Grove, Calif.) at a cross-headspeed of 1 mm/min. The results of the settings times and the DTS testingare recorded in Table II.

Powder x-ray diffraction (XRD) measurements were taken of the samples tomeasure the conversion to HA. The samples were first ground into a finepowder form by hand grinding with the use of a mortar and pestle.Approximately 0.25 grams of sample was placed on the sample holder. Acomputer-controlled powder x-ray diffractometer (Rigaku, Danvers, Mass.)with CuKα radiation generated under the conditions of 40 kv and 25 mAwas used to obtain the XRD patterns. Data were collected in the 2θscanning mode using a step width of 0.01° and count time of 2 sec.

                  TABLE I                                                         ______________________________________                                        Specification of Ingredients                                                                                Median particle                                 Code    Description           size (μm)                                    ______________________________________                                        DCPA    Dicalcium phosphate anhydrous (J T                                                                  11.9                                                    Baker, Inc.) (CaHPO.sub.4)                                            DCPA (g)                                                                              DCPA ground in 95% ethanol in a plane-                                                              0.65                                                    tary mill for 24 h                                                    Ca(OH).sub.2                                                                          Calcium hydroxide (Fisher Scientific Co.)                                                           4.5                                                     (Ca(OH).sub.2)                                                        Ca(OH).sub.2 (g)                                                                      Ca(OH).sub.2 ground in cyclohexane for 6 h                                                          1.36                                            CaO     Calcium oxide (CERAC/PURE, Inc.)                                                                    5.77                                                    (CaO)                                                                 CaCO.sub.3                                                                            Calcium carbonate (J T Baker)                                                                       12.4                                                    (CaCO.sub.3)                                                          CaCO.sub.3 (g)                                                                        CaCO.sub.3 ground in cyclohexane for 16 h                                                           2.55                                            α-TCP                                                                           α-tricalcium phosphate                                                                        12.5                                                    (α-Ca.sub.3 (PO.sub.4).sub.2)                                   α-TCP (g)                                                                       α-TCP ground in cyclohexane for 22 h                                                          2.91                                            ACP     Amorphous calcium phosphate                                                   (Ca.sub.3 (PO.sub.4).sub.2)                                           DCPD    Dicalcium phosphate dihydrate                                                                       7.64                                                    (prepared in our laboratory)                                                  (CaHPO.sub.4.2H.sub.2 O)                                              DCPD (g)                                                                              DCPD ground in 95% ethanol for 24 h                                                                 0.63                                            ______________________________________                                    

                                      TABLE II                                    __________________________________________________________________________    Properties or Cement and Comparative Formulations                                                           Setting    Conversion                           Example                                                                            Solid Components                                                                            Liquid  P/L                                                                              time DTS (MPa)                                                                           to HA                                __________________________________________________________________________         DCPA + Ca(OH).sub.3                                                      1    DCPA (g) + Ca(OH).sub.3                                                                     0.5M Na.sub.2 HPO.sub.4                                                               3  7    3.9                                        2    DCPA (g) + Ca(OH).sub.2                                                                     1M Na.sub.2 HPO.sub.4                                                                 3  5    2.4                                        3    DCPA (g) + Ca(OH).sub.3                                                                     1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              5    3.7 ± .2                                                                         ˜100% HA                       4    DCPA (g) + Ca(OH).sub.2                                                                     5M (NH.sub.4).sub.2 HPO.sub.4                                                         2.5                                                                              5    2.5 ± .2                                                                         90% + DCPA                           5    DCPA (g) + Ca(OH).sub.2                                                                     1N NaOH 2.5                                                                              12   2.6 ± .8                                                                         ˜100% HA                       6    DCPA (g) + Ca(OH).sub.2                                                                     4N NaOH 3  10   3.9                                        7    DCPA (g) + Ca(OH).sub.3                                                                     4N NaOH 2.5                                                                              5    1.5                                        8    DCPA + Ca(OH).sub.2                                                                         1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              8    1.2 ± .2                                                                         HA + DCPA                            9    DCPA + Ca(OH).sub.2                                                                         5M (NH.sub.4).sub.2 HPO.sub.4                                                         3  11   .72                                        10   DCPA + Ca(OH).sub.3                                                                         4N NaOH 3  12   2.3   100% HA                              11   DCPA (g) + Ca(OH).sub.2 (g)                                                                 1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              10   3.9 ± .2                                                                          95% HA                              12   DCPA (g) + Ca(OH).sub.2 (g)                                                                 1N NaOH 2.5                                                                              40   1.6 ± .3                                                                          95% HA                              13   DCPA (g) + Ca(OH).sub.2 (g)                                                                 25 mM H.sub.3 PO.sub.4                                                                2.5                                                                              30                                                   DCPA + CaO                                                               14   DCPA + CaO    1M Na.sub.2 HPO.sub.4                                                                 3  10   1.2 ± 0.2                               15   DCPA + CaO    1N NaOH 3  22                                              16   DCPA + CaO    25 mM H.sub.3 PO.sub.4                                                                3  no setting                                      17   DCPA (g) + CaO                                                                              1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              10   2.0 ± .03                                                                         90% HA                              18   DCPA (g) + CaO                                                                              1N NaOH 2.5                                                                              10   1.7 ± .5                                                                          70% HA                              19   DCPA (g) + CaO                                                                              25 mM H.sub.3 PO.sub.4                                                                2.5                                                                              30                                                   DCPA + CaCO.sub.3                                                        20   DCPA (g) + CaCO.sub.3                                                                       1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              >2h  1.1    10% HA                              21   DCPA (g) + CaCO.sub.3                                                                       5M (NH.sub.4).sub.2 HPO.sub.4                                                         4  >2h  1.6                                        22   DCPA (g) + CaCO.sub.3 (g)                                                                   1M Na.sub.2 HPO.sub.4                                                                 3  20   1.5 ± .2                                                                          99% HA                              23   DCPA (g) + CaCO.sub.3 (g)                                                                   1N NaOH 3  60   1.0 ± .3                                                                          90% HA                              24   DCPA (g) + CaCO.sub.3 (g)                                                                   25 mM H.sub.3 PO.sub.4                                                                3  no setting                                           α-TCP and CaO                                                      25   α-TCP + CaO                                                                           1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              2h         60%                                  26   α-TCP + CaO                                                                           1N NaOH 2.5                                                                              2h   0.8   90%                                  27   α-TCP + CaO                                                                           5M (NH.sub.4).sub.2 HPO.sub.4                                                         2.5                                                                              2h   1.7   95%                                  28   α-TCP + CaO                                                                           2M Citric acid                                                                        2.5                                                                              22   1.6                                        29   α-TCP + CaO                                                                           1N HCl  3  no setting                                      30   α-TCP (g) + CaO                                                                       1N NaOH 3  10   0.9 ± 0.3                               31   α-TCP (g) + CaO                                                                       25 mM H.sub.3 PO.sub.4                                                                3  no setting                                      32   α-TCP (g) + CaO                                                                       1M Na.sub.2 HPO.sub.4                                                                 3  20   0.9 ± 0.4                                    α-TCP + Ca(OH).sub.2                                               33   α-TCP + Ca(OH).sub.2                                                                  1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              no setting                                      34   α-TCP + Ca(OH).sub.2                                                                  1N NaOH 2.5                                                                              no setting                                      35   α-TCP + Ca(OH).sub.2                                                                  5M (NH.sub.4).sub.2 HPO.sub.4                                                         2.5                                                                              >2h  6.5                                        36   α-TCP (g) + Ca(OH).sub.2                                                              1M Na.sub.2 HPO.sub.4                                                                 3  20   1.3 ± .2                                                                         99%                                  37   α-TCP (g) + Ca(OH).sub.3                                                              1N NaOH 3  no setting                                      38   α-TCP (g) + Ca(OH).sub.2                                                              25 mM H.sub.3 PO.sub.4                                                                3  no setting                                      39   α-TCP + Ca(OH).sub.2 (g)                                                              1M Na.sub.2 HPO.sub.4                                                                 3  1 day 0.2                                       40   α-TCP + Ca(OH).sub.3 (g)                                                              1N NaOH 3  no setting                                      41   α-TCP + Ca(OH).sub.3                                                                  1M Na.sub.2 HPO.sub.4                                                                 3  20   2.7 ± .2                                                                         99%                                  42   α-TCP (g) + Ca(OH).sub.2 (g)                                                          1N NaOH 3  20   1.8 ± .5                                                                         99%                                  43   α-TCP (g) + Ca(OH).sub.2 (g)                                                          25 mM H.sub.3 PO.sub.4                                                                3  no setting                                           α-TCP + CaCO.sub.3                                                 44   α-TCP + CaCO.sub.3                                                                    1M Na.sub.2 HPO.sub.4                                                                 3  1 day 1.4                                       45   α-TCP + CaCO.sub.3                                                                    1N NaOH 3  no setting                                      46   α-TCP (g) + CaCO.sub.3                                                                1M Na.sub.2 HPO.sub.4                                                                 3  20   4.8 ± 1.2                                                                        95%                                  47   α-TCP (g) + CaCO.sub.3                                                                1N NaOH 3  20   2.7 ± .7                                                                         95%                                  48   α-TCP (g) + CaCO.sub.3                                                                25 mM H.sub.3 PO.sub.4                                                                3  no setting                                      49   α-TCP + CaCO.sub.3 (g)                                                                1M Na.sub.2 HPO.sub.4                                                                 3  1 day                                           50   α-TCP + CaCO.sub.3 (g)                                                                1N NaOH 3  no setting                                      51   α-TCP (g) + CaCO.sub.3 (g)                                                            1M Na.sub.2 HPO.sub.4                                                                 3  20   7.5 ± .5                                                                         99%                                  52   α-TCP (g) + CaCO.sub.3 (g)                                                            1N NaOH 3  75   2.7 ± .6                                                                         99%                                  53   α-TCP (g) + CaCO.sub.3 (g)                                                            0.2M Na.sub.2 HPO.sub.4                                                               3  15   3.7 ± 1.3                                    ACP + Ca(OH).sub.2                                                       54   ACP + Ca(OH).sub.2                                                                          1M Na.sub.2 HPO.sub.4                                                                 1.3                                                                              18   .14                                        55   ACP + Ca(OH).sub.2                                                                          1N NaOH 1.3                                                                              25   -0                                              Three-Component                                                          56   DCPA (g) + Ca(OH).sub.3 + CaCO.sub.3                                                        1M Na.sub.2 HPO.sub.4                                                                 2.5                                                                              12   3.9   HA + CaCO.sub.3                           DCPD + Ca(OH).sub.2                                                      57   DCPD (g) + Ca(OH).sub.2 (g)                                                                 1M Na.sub.2 HPO.sub.4                                                                 3  10   1.5 ± 0.3                               58   DCPD (g) + Ca(OH).sub.2 (g)                                                                 1N NaOH 3  21   2.0 ± 0.4                               __________________________________________________________________________

Additional data points on DCPD+Ca(OH)₂ cement indicate that DCPD is verysimilar to DCPA in terms of the resulting cement properties. It isbelieved that OCP setting generally be slower. β-TCP is considered to bethe least desirable calcium phosphate salt to be used in the invention.

While the liquid phase that must be used with the new cements may beless biocompatible for certain clinical applications, and at present thestrength of the inventive cement is lower than the best valuesobtainable for the TTCP-containing calcium phosphate cement, the newcalcium phosphate cement is believed highly useful in most applications.The inventive hydroxyapatite cement is sufficiently structurally stablefor reconstruction and augmentation of relatively non-stress-bearingbony tissue, although without augmentation, it may not have sufficientshear-strength resistance to function in the reconstruction ofstress-bearing bones.

The inventive cement may be supplied to the user in a variety of forms,including as powders or as a powder mixture which is later mixed withthe liquid diluent to make putty; or as a pre-mixed putty which maycontain a nonaqueous extender, e.g., glycerin and/or propylene glycol.It may be supplied with or in the instrumentation which is used tointroduce the cement into the body, for example, a syringe, percutaneousdevice, "gun", cannula, biocompatible packet, dentula, reamer, file, orother forms which will be apparent to those of ordinary skill in theart. It is contemplated that the cement, in any of these forms, may bemade available to the surgeon, veterinarian or dentist via a kitcontaining one or more of its key components. The cement is generallyprovided or employed in a sterilized condition. Sterilization may beaccomplished, e.g., by gamma-ray radiation, typically at a dose of 2.5Mrad.

The inventive cement may be employed in a variety of medical, dental,and veterinarian procedures to substitute for missing or defective boneor tooth tissue. For example, it is contemplated that the cements of thepresent invention may be used in place of any of the cements known inthe prior art as: (1) cavity bases and liners to protect the pulp, (ii)materials for capping exposed pulps, (iii) materials to replace orpromote regeneration of bone mineral lost due to periodontal disease,(iv) direct filling materials (may be temporary) that have physicalproperties similar to enamel and are adhesive to enamel and dentin, (v)a cement to build up alveolar ridges in edentulous patients, (vi) anendodontic filling material for root canals, (vii) a material to cementretention pins, (viii) a material for filling sockets after a toothextraction, (ix) a replacement of bone that has been removed surgicallyor lost due to trauma, (x) a cement for implanting or replanting teeth,(xi) a luting cement in dentistry and orthopedic surgery, (xii) aninvestment mold material, (xiii) a material which will promote bonemineral in its vicinity, (xiv) a remineralizing polish for use in placeof pumice, and (xv) a root cement for remineralizing and desensitizingof exposed root surfaces, (xvi) a cement for orthopedic prostheses,(xvii) a tooth implant, (xviii) a device for percutaneous passage oftubes, wires and other medical instruments through the skin, (xxix) areplacement material for bone loss due to abscess and (xxx) aself-setting cement for binding non-self-setting calcium phosphatebiomaterials such as porous or non-porous HA and P-tricalcium phosphate.Reconstruction of cleft palate and other congenital skeletal defects iscontemplated with use of the inventive cement, as are other forms ofreconstructive and cosmetic surgery.

Various additives may be included in the inventive cements, slurries andpastes to adjust their properties and the properties of thehydroxyapatite products made from them. For example, proteins,medicaments, supporting or strengthening filler materials, crystalgrowth adjusters, viscosity modifiers, pore forming agents,osteoinductive factors such as demineralized bone, bone morphogenicproteins and other additives may be incorporated without departing fromthe scope of this invention. The referenced filler materials may includenon-toxic biocompatible natural or synthetic polymers, non-toxicbiocompatible metals such as titantium mesh, or other non-toxicbiocompatible organic and inorganic materials. These fillers can be inthe form of granules, fibers, rods, sheets or grids. These fillermaterials enhance the strength of the cement.

In addition, the referenced pore forming agents create pores or channelssufficiently large to cause vascularization of tissue which infiltratesthe cement once placed in the body. Such pore forming agents arepreferably substantially insoluble in the cement and can be removed byeither resorbsion into body tissue, dissolution in physiological fluids,dissolution in solvents or heating after the cement has hardened. Thepore forming agents may include sugar, sodium bicarbonate and phosphatesalts. The phosphate salts are effective pore forming agents because theuse of concentrated phosphate solutions in the cement makes themrelatively insoluble in the cement. A particularly useful phosphate saltis disodium phosphate.

An additive of particular significance in dental applications isfluoride containing compounds. When fluoride salts such as NaF, CaF₂,SnF₂, Na₂ PO₃ F or Na₂ SiF₆ are added in sufficient quantity theyincrease the rate of formation of HA and fluorapatite. Preferably, thecalcium phosphate cements will have a fluoride content of about 3.8%.

Other especially useful additives are seed crystals of HA orfluorapatite and gelling agents. Adding a sufficient quantity ofcrystals of HA or fluorapatite to the cement increases the rate ofconversion to HA and thus reduces the setting time of the cement.Gelling agents such as hydroxypropyl methylcellulose, carboxylmethylcellulose, starch, proteoglycans and glycoproteins have the effectof causing more rapid hardening of the cement.

The novel implants thus prepared are also contemplated as part of thepresent invention. Where such implants contact living bone tissue, it isbelieved that the lack of fusion of the hydroxyapatite crystallitesresulting from the setting reaction of the inventive cement will allowosteoclasts to slowly resorb the implants over time.

While the primary benefits of the present invention are believed torelate to dental, medical and veterinary applications, it is alsocontemplated that the techniques may be employed in conjunction with anindustrial hydroxyapatite cement, for example, to repair damage, e.g.,from corrosion, to marble and other stone articles.

It should be understood that the foregoing disclosure emphasizes certainspecific embodiments of the invention and that all modifications oralternatives equivalent thereto are within the spirit or scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A calcium phosphate-containing composition, whichself-hardens to hydroxyapatite as the predominant product at ambienttemperature, comprising a calcium phosphate salt that is substantiallyfree of tetracalcium phosphate, an additional source of slightly solublecalcium and an aqueous solution having a pH of about 12.5 or above.
 2. Acalcium phosphate-containing composition, which self-hardens tohydroxyapatite as the predominant product at ambient temperature,comprising a calcium phosphate salt that is substantially free oftetracalcium phosphate, an additional source of slightly soluble calciumand an aqueous solution having a concentration of phosphate of about 0.2mol/L or above, in the absence of solid crystalline phosphoric acid. 3.A calcium phosphate tissue substitute precursor having as a calciumphosphate component the composition of claim
 1. 4. A calcium phosphatetissue substitute precursor having as a calcium phosphate component thecomposition of claim
 2. 5. A calcium phosphate bone cement whichself-hardens at ambient temperature and is resorbable when implanted incontact with living bone comprising the composition of claim
 1. 6. Acalcium phosphate bone cement which self-hardens at ambient temperatureand is resorbable when implanted in contact with living bone comprisingthe composition of claim
 2. 7. A biological implant comprised of thecement of claim 5 inserted into contact with living tissue.
 8. Abiological implant comprised of the cement of claim 6 inserted intocontact with living tissue.
 9. A method of bonding living tissuecomprising the step of inserting the tissue substitute precursor ofclaim 3 into the tissue to form a bridging agent between the tissue tobe bonded.
 10. A method of bonding living tissue comprising the step ofinserting the tissue substitute precursor of claim 4 into the tissue toform a bridging agent between the tissue to be bonded.
 11. A method ofaugmenting living tissue comprising the step of inserting the tissuesubstitute precursor of claim 3 into contact with the living tissue. 12.A method of augmenting living tissue comprising the step of insertingthe tissue substitute precursor of claim 4 into contact with the livingtissue.
 13. A method for repairing bony defects comprising the step offilling the defect with the tissue substitute precursor of claim
 3. 14.A method for repairing bony defects comprising the step of filling thedefect with the tissue substitute precursor of claim
 4. 15. A kit forpreparing a calcium phosphate-containing composition comprising (1) acalcium phosphate salt which is substantially free of tetracalciumphosphate, (2) an additional source of slightly soluble calcium and (3)an aqueous solution having a pH of about 12.5 or above.
 16. A kit forpreparing a calcium phosphate-containing composition comprising (1) acalcium phosphate salt that is substantially free of tetracalciumphosphate, (2) an additional source of slightly soluble calcium, and (3)an aqueous solution having a phosphate concentration of about 0.2 mol/Lor above, in the absence of solid crystalline phosphoric acid.
 17. Animplant constructed in vitro for reconstruction of bone or dental tissuecomprising a mixture of (1) a calcium phosphate salt that issubstantially free of tetracalcium phosphate, (2) an additional sourceof slightly soluble calcium, and an aqueous solution having a pH ofabout 12.5 or above, and having a high phosphate concentration, in theabsence of solid crystalline phosphoric acid whereby the mixtureself-hardens to hydroxyapatite as the predominant product at ambienttemperature and can be shaped as desired.
 18. A calciumphosphate-containing composition according to claim 1, wherein theadditional source of slightly soluble calcium is selected from the groupconsisting of calcium carbonate, calcium oxide and calcium hydroxide.19. A calcium phosphate-containing composition according to claim 2,wherein the additional source of slightly soluble calcium is selectedfrom the group consisting of calcium carbonate, calcium oxide andcalcium hydroxide.
 20. A kit according to claim 15, wherein theadditional source of slightly soluble calcium is selected from the groupconsisting of calcium carbonate, calcium oxide and calcium hydroxide.21. A kit according to claim 16, wherein the additional source ofslightly soluble calcium is selected from the group consisting ofcalcium carbonate, calcium oxide and calcium hydroxide.
 22. An implantaccording to claim 17, wherein the additional source of slightly solublecalcium is selected from the group consisting of calcium carbonate,calcium oxide and calcium hydroxide.